Encoding Apparatus, Encoding Method, and Computer Product

1. An encoding apparatus that compresses a stereo signal using a sum signal and a difference signal of a left component signal and a right component signal of the stereo signal, comprising: a transforming unit configured to transform a left spectrum signal and a right spectrum signal to produce a sum signal and a difference signal; a comparing unit configured to compare a value indicative of an output of the difference signal with a threshold for each frequency band; a correcting unit configured to correct the value to zero when the value is lower than the threshold; a calculating unit configured to calculate complexity of the sum signal and complexity of the difference signal; a setting unit configured to set, based on the complexity, an allocation rate of bits to be allocated in quantizing the sum signal and the difference signal; and a quantizing unit configured to quantize the sum signal and the difference signal based on the allocation rate, wherein the calculating unit includes: an error and power calculating unit responsive to the transforming unit and the correcting unit for producing admissible error signals and electric power signals, a perceptual entropy value calculating unit responsive to the error and power calculating unit for calculating first and second values indicative of perceptual entropy of the sum signal and the difference signal, and the complexity is calculated based on the first and second calculated values.

2. The encoding apparatus according to claim 1 , wherein the setting unit is configured to set the allocation rate so as to allocate a predetermined number of bits to each frame of the sum signal and the difference signal, the frame time-divided at a predetermined interval.

3. The encoding apparatus according to claim 1 , wherein the setting unit is configured to set the allocation rate low for a signal having low complexity, and to set the allocation rate high for a signal having high complexity at a time of quantization by the quantizing unit.

4. The encoding apparatus according to claim 1 , wherein, the complexity is calculated based on a ratio between the first and second calculated values.

5. The encoding apparatus according to claim 1 , wherein the complexity is calculated based on a difference between the first and second calculated values.

6. The encoding apparatus according to claim 1 , wherein the calculating unit is configured to calculate the first and second calculated values at all frequency bands of the sum signal and the difference signal, and the complexity is calculated based on an average of the first and second calculated values calculated at all frequency bands.

7. The encoding apparatus according to claim 1 , wherein the calculating unit is configured to calculate the first and second calculated values at all frequency bands of the sum signal and the difference signal, and the complexity is calculated based on a sum of the first and second calculated values calculated at all frequency bands.

8. An encoding method in which a stereo signal is compressed using a sum signal and a difference signal of a left component signal and a right component signal of the stereo signal, comprising: transforming a left spectrum signal and a right spectrum signal to produce a sum signal and a difference signal; comparing a value indicative of an output of the difference signal with a threshold for each frequency band; correcting the value to zero when the value is lower than the threshold; calculating complexity of the sum signal and complexity of the difference signal; setting, based on the complexity, an allocation rate of bits to be allocated in quantizing the sum signal and the difference signal; and quantizing the sum signal and the difference signal based on the allocation rate, wherein the step of calculating the complexity includes: producing admissible error signals and electric power signals in response to the step of transforming and the step of correcting, and calculating first and second values indicative of perceptual entropy of the sum signal and the difference signal in response to the admissible error signals and the electric power signals, and the complexity is calculated based on the first and second calculated values.

9. A computer-readable recording medium that stores therein a computer program for realizing an encoding method in which a stereo signal is compressed using a sum signal and a difference signal of a left component signal and a right component signal of the stereo signal, the computer program making a computer execute: transforming a left spectrum signal and a right spectrum signal to produce a sum signal and a difference signal; comparing a value indicative of an output of the difference signal with a threshold for each frequency band; correcting the value to zero when the value is lower than the threshold; calculating complexity of the sum signal and complexity of the difference signal; setting, based on the complexity, an allocation rate of bits to be allocated in quantizing the sum signal and the difference signal; and quantizing the sum signal and the difference signal based on the allocation rate, wherein the step of calculating the complexity includes: producing admissible error signals and electric power signals in response to the step of transforming and the step of correcting, and calculating first and second values indicative of perceptual entropy of the sum signal and the difference signal in response to the admissible error signals and the electric power signals, and the complexity is calculated based on the first and second calculated values.